PROJECT SUMMARY Rapid eye movement (REM) sleep is characterized by activated electroencephalogram (EEG) and skeletal muscle paralysis and is associated with vivid dreaming. Disturbances in REM sleep are symptomatic of mood disorders and are considered as a biomarker for depression. Core circuits generating REM sleep are located in two brainstem areas, the pons and medulla. The role of the pons in REM sleep regulation is well studied, while the importance of the medulla has only recently been appreciated. Previous studies, including our preliminary data, suggest an important role of neurons in the dosomedial medulla (dmM) in REM sleep control. However, little is known about the molecular identity of neurons in the dmM that promote REM sleep, about their activity during sleep and their downstream targets. To fill this gap, we will employ state-of-the-art tools for circuit mapping and methods for recording and perturbing neural activity in the mouse model. Our long-term objective is to unravel the circuit mechanisms controlling REM sleep. Our preliminary results show that a genetically defined population of neurons in the dmM that express corticotropin-releasing hormone (CRH) promotes REM sleep. The central objective of this proposal is to understand how the activity of dmM CRH neurons controls REM sleep and identify the downstream targets in the pons through which they induce REM sleep. In Aim 1, we will manipulate the activity of CRH neurons using opto- and chemogenetic approaches and examine how activation and inhibition affect the initiation and maintenance of REM sleep. Using optrodes, we will record the activity of dmM CRH neurons in freely moving mice across multiple periods of wakefulness, NREM, and REM sleep and test whether their activity pattern supports a role in initiating and maintaining REM sleep. In Aim 2, we will investigate whether the axonal projections from the dmM to downstream areas in the pons mediate the effects on REM sleep. We will use pseudo-typed rabies viruses for circuit mapping and employ in vivo calcium imaging and optogenetic perturbations to examine the role of this pathway. The proposed studies are innovative, as they will reveal the role of an unexplored brainstem circuit in REM sleep control, and the results will extend existing models of how neurons in the medulla interact with post-synaptic areas in the pons to regulate REM sleep. The capability to manipulate REM sleep by targeting a genetically defined population of medullary neurons will provide a powerful tool to understand the pervasive link between REM sleep and mood disorders.